Thermal Runaway

What It Is and How to Prevent It

What is Thermal Runaway?

In simplest terms, thermal runaway begins when the heat generated within a battery exceeds the amount of heat that is dissipated to its surroundings.  If the cause of excessive heat creation is not remedied, the condition will worsen.  Internal battery temperature will continue to rise, causing battery current to rise, creating a domino effect.  The rise in temperature in a single battery will begin to affect other batteries in close proximity, and the pattern will continue, thus the term “runaway.”

Resulting damage to a battery cabinet due to battery overheating.

The Dangers of UPS Battery Overheating

Left uncorrected, internal temperature will continue to increase, causing battery overheating and the outer casing to bulge, melt and/or rupture. When the casing of a VRLA battery becomes compromised, hydrogen sulfide gas (smelling like rotten egg) will escape. If this is observed, charging power to the battery needs to be removed immediately. If undetected, thermal runaway can lead to catastrophic results, including fire, explosion, sudden system failure, costly damage to equipment, and possibly personal injury.

Thermal runaway can occur in lithium ion batteries and VLA, but this discussion focuses mainly on VRLA batteries, since they are most common in the industry.

The remains of a battery cabinet suffering thermal runaway.

What causes the battery overheating that leads to thermal runaway?

There can be many contributing factors to thermal runaway in your UPS battery:

  • Ambient Temperature – Battery life is compromised when continually subjected to ambient temperatures above 77° F (25° C).  A high ambient temperature reduces a battery’s ability to shed heat. This leads to an increase of the internal chemical reaction and increased float charging current further increasing the battery temperature.Battery life is compromised when continually subjected to ambient temperatures above 77° F (25° C).  A high ambient temperature reduces a battery’s ability to shed heat. This leads to an increase of the internal chemical reaction and increased float charging current further increasing the battery temperature.
  • Age of the Battery - Batteries that are near end of life (commonly defined as a reduction to 80% of rated capacity) have been degrading over time, possibly requiring longer time to charge and/or being subjected to higher current and internal heat generation.
  • Float Charging Voltage – Excessive float charging voltage can cause an increase in float charging current and increased internal battery temperature.
  • Overcharging – Continuous overcharging will damage battery internals, leading to a shortened battery life.

 

A short circuited cell has left its mark.

How to Prevent Thermal Runaway

Be on the alert for other battery anomalies, such as ground faults and shorted cells, which can negatively impact charging voltages and ultimately lead to thermal runaway. If you notice a battery overheating, disconnect it from charge and inspect it for other damage. Separate it from other systems to avoid runaway.

The best way to avoid the cataclysmic event that thermal runaway can become is through the application of regularly scheduled preventive maintenance of the batteries, continuous monitoring of battery cabinet temperature and/or individual battery terminal temperature, and systematic review of the historic battery data by a skilled technician.

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Photos on this page provided by Critical Power Solutions Inc. and C & C Power, Inc.